The subject invention relates to feeding of single sheets of paper or the like from a stack of sheets for processing by folders, printers, copiers or the like. More particularly, it relates to detecting double fed sheets which occur when a sheet feeder fails to properly singulate sheets from the stack.
In printers, copiers, inserters, and similar such systems it is frequently necessary to singulate sheets from a stack of sheets for further processing by the system. Many mechanisms have been developed to perform this singulation function, and, in general, they are effective. However, inevitably such sheet feeders will fail and feed a "double" (i.e. two or more overlapping sheets). Such double fed sheets may jam in the system, requiring operator intervention to clear the jam. Perhaps more importantly, if the sheets contain information or are otherwise unique (e.g. return of cancelled checks) then their destruction in a jam caused by a double feed may significantly interfere with operations.
For these reasons it is known to provide such systems with detectors down stream from the sheet feeder to detect double fed sheets before a jam and possible destruction of the sheets can occur. One known method is to use an optical system to measure the transparency of a sheet after it is fed from the sheet feeder. Another known method uses precise, sensitive mechanical switches to detect an increase in the thickness of a fed sheet. Both of these methods for detecting double fed sheets involve precise, pains staking adjustments each time the type of sheet to be fed is changed.
Accordingly, it is an object of the subject invention to provide a method and apparatus for detecting double fed sheets which is easily adaptable to different types of sheets to be fed.